Sept. 11 always brings Barcelonans into the streets to dance the sardana, sing for their enemies’ blood in the anthem “Els Segadors” and chant political slogans in celebration of their national holiday, the Diada de Catalunya. But this year, a new intensity colored the Catalans’ nationalist fervor. The independence movement’s flag bearing a white star against a blue triangle outnumbered the region’s official yellow-and-red-striped standard. A pro-independence march, which in the past has never drawn more than 50,000 people, pulled in a crowd estimated by city police at 1.5 million. And every newspaper in the city carried the results of a poll released this week that reveals a once unimaginable transformation: half the population of Catalonia supports secession from Spain.

“We have no other option since our will has been totally ignored” says Soledat Balaguer, a member of the secretariat of the Catalan National Assembly, organizers of the demonstration that shut down the city center. “Catalonia needs to be its own state.”

It wasn’t always thus. Unlike the Basque Country, where support for independence from Spain has run high for generations, most Catalans have traditionally favored greater autonomy over outright secession. As late as 2010, a poll conducted by Catalonia’s Center for Opinion Studies found that only 25.2% of the population favored independence. That number had more than doubled in its latest survey, released this week, which found a historic high of 51.1% wanting out of Spain. How to explain the dramatic change?

One factor amplifying pro-independence sentiment in recent years was the Spanish state’s legal challenge to a 2006 statute, approved in a Catalonia-wide referendum, that transferred significant powers to the regional government. When Spain’s highest court declared many of the statute’s provisions unconstitutional, “it outraged Catalans,” says Montserrat Guibernau, professor of politics at Queen Mary, University of London. “That initiated the change in thinking.”

But the recent surge in secessionist support is closely tied to Spain’s economic crisis. Although Catalonia is the wealthiest region in Spain, it is also the most heavily in debt, running a fiscal deficit of 8%. Two weeks ago, it requested a 5 billion euro bailout from Spain’s central government, a request that prompted the president of the Extremadura region to complain that those funds would come “from the pockets of all Spaniards.” But in the minds of many Catalans, the region was simply asking for its own money to be fairly returned.

Under the current fiscal system, Catalonia collects taxes from its residents, but turns them over to the central government, which then disburses a designated amount to each region to pay for public salaries, social services, infrastructure and the like. In 2009, the latest year for which figures are available, Catalonia provided 19.49% of the federal government’s tax revenue, yet received only 14.03% of the state’s spending.

It is that discrepancy, says Catalan President Artur Mas, that explains the region’s deficit. Mas has called for a fiscal reform that would enable his government to collect its own taxes and turn over a designated amount to the central state (rather than the other way around). So potent is the popular sense of injustice that even Mas, whose Convergència i Unió party has never been pro-independence, hinted that his party’s stance may change if it does not achieve the reform it seeks. “If we cannot reach a financial agreement,” he told the BBC today, “the road to freedom for Catalonia is open.”

The economic crisis has also increased pro-independence sentiment in less concrete ways. Historian Enric Ucelay-Da Cal, who specializes in Catalan nationalism, sees the growth in secessionism as a reactionary response to the erosion of many of Catalonia’s traditionally distinguishing features. As small businesses — once a mainstay of the Catalan economy — find it harder to survive, and young people find it increasingly difficult to find jobs, independence is viewed as a panacea. “There’s a perception that once we get ‘our’ money back and go back to the good old days of ‘our’ culture, then everything will be all right again,” Ucelay says.

On Tuesday afternoon, it seemed as if everything was. As hundreds of thousands of people wrapped themselves in independence flags and danced their way down the Via Laietana and the Gran Via, the mood was markedly festive. But a darker reality lay just beneath the surface. “There’s terrible pressure on both Spain and Catalonia right now, and the frustration is enormous,” says Guibernau. “Secession was always unthinkable in Catalonia. But it’s precisely at moments like this that the unthinkable can occur.”